1. Field of the Invention
The present invention relates to a semiconductor device containing gallium nitride (GaN) -based compound semiconductor.
2. Related Art
Gallium nitride-based (GaN-based) semiconductors have wide band gaps, and the characteristics of GaN-based semiconductors have been used in research and development of high-brightness ultraviolet to blue/green LEDs and violet laser diodes. Further, high-frequency and high-power GaN transistors or the like have been fabricated.
In a GaN-based semiconductor, since an effective mass of an electron or a positive hole is larger than that of a GaAs-based semiconductor, a transparent carrier density of the GaN-based laser is larger than that of GaAs-based laser. Therefore, a threshold current density of a GaN-based laser is inevitably higher than that of a GaAs-based laser. A representative value of the threshold current density of the GaN-based laser is about 1 to 3 kAcm−2.
As described above, since a GaN-based laser has a high threshold current density, it is critically important to suppress overflow of carriers (particularly electrons). In a GaN-based laser, a GaAIN layer doped with p-type impurity is often disposed near an active layer to suppress overflow of electrons (Shuji Nakamura et al., “InGaN-Based Multi-Quantum-Well-Structure Laser Diodes”, Japanese Journal of Applied Physics, Jan. 15, 1996, volume 35, No. 1B, pp. L74-L76, M. Hansen et al., “Higher efficiency InGaN laser diodes with an improved quantum well capping configuration”, Applied Physics Letters, Nov. 25, 2002, volume 81, No. 22, pp. 4275-4277).
However, during crystal growth of an actual device structure, InGaN and GaN/GaAlN used as guide layer materials are grown at different temperatures. The growth temperature of InGaN is about 700 to 800° C., whereas the growth temperature of GaN/GaAIN is 1000 to 1100° C. In other words, after InGaN is grown, the growth is suspended, InGaN undergoes a temperature rising process, and then GaN/GaAIN is grown. It has been found that a defect caused by heat damage is introduced to a crystal growth layer in this temperature rising process. When the layer with such a defect is arranged close to an active layer, the life of the device may decrease. Therefore, in order to achieve a highly reliable device, it is important to locate the layer with such a defect away from the active layer.
When a GaAIN layer doped with p-type impurity is arranged quite close to an active layer, the p-type impurity causes a free carrier loss and, on the contrary, increases a threshold current density. Further, the p-type impurity may diffuse to the active layer. In this case, the loss increases and the threshold current density also increases. Even if the diffusion of p-type impurity to the active layer is suppressed in the initial stage of energization of the laser diode, the impurity may diffuse to the active layer during a life test with a constant optical output, so that the threshold current density may increase and the laser diode may be finally disabled. In this way, the diffusion of p-type impurity to the active layer is a serious problem to the reliability of the device.